1. Field of the Invention
This invention relates to a method of making a target for sputtering, and more particularly to a method of making a magnetic film target for use in sputtering.
2. Description of the Prior Art
In general, the sputtering method can form denser films than the deposition method. The film formed by the sputtering method shows high adhesion to a substrate. However, the sputtering method is disadvantageous in that the film forming rate is relatively low. For this reason, the method has been used mainly to form films on a laboratory scale for the purpose of testing and research. For film forming on an industrial scale, the method has been used only to a limited extent. Recently, however, there have been developed several types of so-called high speed sputtering methods that can form films at a high speed. Such high speed sputtering methods are now widely used for film forming on an industrial scale.
Among the high speed sputtering methods, the most attractive is generally called "magnetron sputtering". In the magnetron sputtering method, a means for generating a magnetic field, such as permanent magnet and electromagnet, is located to the rear of the target. This magnetic field generating means generates, in the vicinity of the target surface, a magnetic field which intersects perpendicularly to the electric field applied between the target and a substrate holder. This magnetic field confines electrons necessary for gas ionization in the vicinity of the target surface and causes electrons to turn toward the target surface. As a result, electrons move a longer distance and the collision probability of electrons with gas molecules is increased, whereby the gas ionization efficiency is improved. Thus, in the magnetron sputtering method, the gas ionization efficiency is enhanced by applying the magnetic field so as to intersect perpendicularly with the electric field. Therefore, the sputtering efficiency is increased, resulting in a higher film forming rate.
However, magnetron sputtering of this type is disadvantageous in that magnetic materials cannot be used as the target. If a magnetic material is used as the target in the magnetron sputtering method, the lines of magnetic force generated by the magnetic field generating means are not emitted from the target surface but pass inside the target body. In this case, the gas ionization efficiency is identical with that of the ordinary sputtering method using no magnetic field and, accordingly, the film forming rate cannot be improved. To solve this disadvantage of the magnetron sputtering method, it is known to reduce the thickness of the magnetic material target and increase the magnetic reluctance thereof. With this approach, the lines of magnetic force generated by the magnetic field generating means at the rear of the target can go through the target and are emitted from the target surface. Accordingly, a magnetic field can be formed in the vicinity of the target surface. With respect to the thin magnetic material target used in the magnetron sputtering method, a target thickness of 1.5 mm or less has been reported. Such a target is sometimes prepared by plating. However, when the target is prepared in a plating solution, the obtained target contains impurities and, in general, it is impossible to prepare high-quality targets.
However, it is not easy to prepare thin targets having a large area. In general, when preparing a thin magnetic material target, a sheet-shaped slice is first cut off from a large mass of a target material. The sheet-shaped slice obtained is then ground and polished to form a thin film. The film thus formed is thereafter bonded to a substrate (holder) made of a metal such as copper, which serves to cool (water-cool) the film and prevents the film from being distorted. However, this method of preparing a target involves many steps such as slicing off, grinding and polishing the sheet-shaped slices, bonding the formed films to substrates, and the like. Thus this method requires a complicated production process, and the target products become very expensive. Further, in general, film targets obtained by this method show uneven film thickness. In addition, the adhesion strength of films to substrates is so low that the film targets often separate from the substrates and cannot be used any more. Furthermore, it is sometimes necessary to prepare a target having a composition which changes in the thickness direction thereof, instead of an ordinary target in which the composition is the same throughout the target body. However, the above-described conventional method cannot prepare targets in which the composition changes in the thickness direction.
For the reasons described above, there is a need for a method of preparing a magnetic film target for use in the sputtering process, particularly in the magnetron sputtering process, which can eliminate the disadvantages by the above-described conventional method.